I am reminded of that old saying, “A son is a son until he takes a wife, a daughter is a daughter for the rest of your life”.
I empathize with your pain, at the same time…you cannot and will not be able to control the choices your adult children will make, especially after they choose a partner. It seems like your husband recognizes the autonomy your son has and the fact your son has chosen to make his partner the number one person in his life, not his mother. This says more about how much he values his partner, not really anything about his love for you.
I, personally, think it is very important in spouse/partner relationships for the spouses to be each other’s #1, and not to have ‘competing’ #1 loyalty to either set of parents. My best advice would be not to try to make this a test of family love or loyalty going forward.
It would be generous if your son and his girlfriend/wife/partner wanted to visit you on Christmas Day. But, if he chooses not to - you need to figure out how to make the holiday nice and pleasant for those who are there on that day. Our children are going to live their own lives and make new traditions with the people they choose to build their lives with, it is not reasonable to expect that those decisions will be made with your wishes and desires in mind. It would be nice if they were, but that should never be your expectation.
Also, while it might be tempting to tell your son about your tears, I would highly recommend not doing this. If you are thinking about ever opening the topic with his girlfriend - STOP! The most I would ever recommend saying to the two of them would be something along the lines of, “Please know that while I understand you’ve chosen to spend Christmas at (insert partner’s name) this year, you both are always welcome and wished for at our home as well!” Please only say this if you can say it with a smile and good cheer and NOT with pleading voice and/or tears.
If this girlfriend ends up becoming your dil, the mother of your grandchildren…she will be the person really in charge of what kind of/how much of a relationship you have with your grandchildren until they are old enough (teens) to navigate their own relationship with you. Your son has already told you he intends to defer to her wishes to spend these holidays with her family. My best advice is to make it easy for them to choose to be with you when they want to be with you. And to realize that guilt won’t make your son’s partner feel badly that she brought her partner to her family’s holiday celebration. It will make her feel like she successfully avoided dealing with overly clingy, demanding potential mil. And it will affect how she sees any future responses you may have to the choice they as a couple make (in terms of wedding plans, children, etc).
It’s not easy being the mother of the son in these circumstances. But it is normal for sons to allow their wives to choose who they spend holidays with and how holidays are celebrated. You can work on being an ally to your son’s girlfriend (which may mean swallowing a lot of what you really feel when asked for thoughts and opinions - which is hard to be sure) and build a relationship where seeing you and your family can become a priority added to their lives OR you can try to act like a parental figure who deserves equal billing to her parents (a lot less successful strategy which can end up being even more frustrating than the first). I realize neither of those options probably sound all that appealing when you are missing your baby. So, hugs to you on Christmas, I wish you love and strength. And open the presents with the son & family who are there. It will be fun!